During development, dendrites and dendritic spines form and turn over dynamically. In adult brains, however, most dendrite branches and many dendritic spines are stable. Defects in dendrite arbor and dendritic spine stability underlie numerous psychiatric and neurological diseases, including late-onset disorders such as schizophrenia, Major Depressive Disorder, and Alzheimer's disease. I provide evidence in this proposal that loss of the extracellular matrix protein laminin ?5 specifically from excitatory neurons disrupts spine stability, causes dendrite regression during adolescence, and compromises normal synaptic transmission and animal behavior. In my research plan, I propose to identify which synaptic partner produces the necessary laminin ?5, determine when it functions to stabilize dendritic structure and synaptic transmission, and test whether activity changes cause spine stability disruption found in laminin ?5 knockout neurons. Aim 1. To elucidate where the ?5-containing laminin is produced and when it is necessary. My preliminary data show that loss of laminin ?5 specifically from excitatory forebrain neurons causes dendrite loss and synaptic dysfunction in CA1 neurons starting after P21. I also show that adult excitatory neuron- specific laminin ?5 KO mice lack laminin ?5 protein specifically near synapses. Which synaptic partner provides this laminin is a fundamental and unresolved question. Knowledge of its source is critical to understanding how its expression, processing, and secretion are controlled, and ultimately what factors govern dendritic stability. To address this, I will selectively inactivate the lama5 gene in presynaptic (CA3) or postsynaptic (CA1) cells and then measure dendritic arbors, dendritic spine density, and synaptic currents in the postsynaptic neuron. Another critical question is when laminin ?5 functions to control these phenotypes. To determine this, I will use inducible genetic inactivation of laminin ?5 at time points before, during, and after adolescence and then measure dendrite arbors, dendritic spines, and synaptic currents. Aim 2: To determine whether synaptic transmission defects drive dendritic spine destabilization in laminin ?5 knockout neurons. My preliminary studies indicate that acute hippocampal slices from excitatory- specific laminin ?5 knockout mice exhibit increased currents at CA3:CA1 synapses beginning after P21. I also find cultured laminin ?5 KO neurons exhibit decreased spine density, increased spine head width, and increased spine size fluctuations relative to WT neurons. These phenotypes can all be rescued with application of exogenous ?5-containing laminin. A fundamental question that arises from these studies is whether the increased currents at laminin ?5 KO synapses drive the loss of dendritic spine stability. To test this possibility, I will use calcium imaging to test whether rescue with exogenous v5-containing laminin attenuates calcium transients before rescuing spine fluctuation and also whether restoring WT activity levels restores normal spine fluctuation, density, and morphology in laminin ?5 KO neurons.